Adding support for HTTP/2 binary headers

Motivation:

The HTTP/2 spec does not restrict headers to being String. The current
implementation of the HTTP/2 codec uses Strings as header keys and
values. We should change this so that header keys and values allow
binary values.

Modifications:

Introduced ByteString from okio (with a few minor mods). Added new
BinaryHeaders interface and made Http2Headers extend it. Various changes
throughout the HTTP/2 codec to use the new interface.

Result:

HTTP/2 codec no longer requires string headers.
Need to release `frame` here. See https://github.com/netty/netty/blob/master/codec-http2/src/test/java/io/netty/handler/codec/http2/DefaultHttp2FrameIOTest.java#L261 for example.